Opto-electronic devices, such as semiconductor lasers and SLEDs provide the light source for different aspects of fiber-optic telecommunication systems. Opto-electronic radiation source devices, such as lasers, having wavelength outputs ranging from 1.31 to 1.55 microns (μm) are the most commonly used lasers for telecommunication operation. Radiation source devices with wavelengths of 1.31 μm are typically used for short distance transmission and cable TV signals, while wavelengths of 1.42–1.6 μm are typically used for fiber optic communications. Lasers having wavelengths in the range of 1.42 to 1.48 μm are typically used as pump lasers for Raman and Erbium Doped Fiber Amplifiers (EDFA). In long distance transmission one desires to use, for example, lasers having wavelengths between 1.54 to 1.56 μm. For all telecom applications and especially for EDFA pump sources, operating currents for semiconductor sources exceed by more than an order of magnitude the threshold current.
Essentially, as one can ascertain from the prior art, a weak temperature dependence of the main laser characteristics is extremely important for telecom lasers. As one knows in the prior art, it is not easy to keep high performance at elevated temperatures for lasers operating in the wavelength range of 1.3–1.5 μm. A widely used material for telecom lasers is InGaAsP compounds grown on InP substrates. Numerous investigations of InGaAsP/InP lasers have demonstrated that the strong temperature degradation of their parameters is partially associated with insufficient high energy level barriers for confinement of the electrons in the laser active region. As a result of the low barrier in the conduction band, some of the electrons injected from n-doped cladding layer passes through the active region and are lost in the p-doped cladding layer. There are many investigators that have looked at this problem, and people have provided various solutions to decrease electron leakage at elevated temperatures. For example, see an article entitled, “Lasing Characteristics Under High Temperature Operation of 1.55 μm Strained InGaAsP/InGaAlAs MQW Laser With InAIAs Electron Stopper Layer” by H. Murai et al., published in Electronics Letters on Nov. 23, 1995, Volume 31, Number 24. In that paper there is shown a 1.55 μm strained InGaAsP/InGaAlAs MQW laser in which InGaAIAs cladding layers and InAIAs electron stopper layer have been incorporated to reduce electron leakage current. The device showed low threshold current with high maximum temperature of CW operation. As indicated in the paper, superior lasing characteristics were demonstrated through comparison with conventional strained MQW lasers without electron stop layer. A similar design was used for the fabrication of 1.3 μm lasers that could operate without thermoelectric coolers. Such lasers are required for subscriber networks and optical interconnection systems. See, for example, an article entitled, “High Temperature Operation of AlGaInAs Ridge Waveguide Lasers With a p-AlInAs Electron Stopper Layer” published in Japanese Journal of Applied Physics on pages 1230 through 1233 by Takemasa et al.
As can be seen in these papers, there are band diagrams that show the active region of the laser including Quantum Wells (QW) sandwiched between layers of materials with a band gap larger than that of the active region. Thus energy barriers confining the injected electrons and holes in the active region are created. The prior art utilized InGaAsP and InP to create an active region and barriers for lasers operating in the 1.2 to 2.0 μm range. The disadvantage of these material is that the energy barriers for electrons is lower than that for holes and when one wants to inject electrons to generate photons in the QWs, there is a leakage of electron current from active region to the p-doped InP cladding layer that limits laser performance. This leakage current component is superlinearly related to total current and temperature sensitive, in that as the leakage current increases at a rate greater than an increase in the total current. The problem is more severe for lasers operating at 1.3 μm than for lasers that operate at 1.55 μm. In fact, the problem is so severe that one employs AlInGaAs as a cladding layer material or AlInAs as a stop-electron (blocking) layers.
However, aluminum compounds are not desirable for telecommunication devices because aluminum easily oxidizes and thus creates problems for laser fabrication as well as for long term reliability. The prior art has attempted to solve this problem by using a InGaP large band gap material as a blocking layer. The additional problem for InGaAsP/InP telecom lasers arises at operation at high current densities. Measurements of output power versus current (P-I characteristics) at both short pulsed regime and continuous wave (CW) regime demonstrate the transition from a linear performance to a sublinear performance at high current density. This phenomenon is called the P-I characteristic rollover or saturation effect. In the short pulse regime, where device heating is negligible, the saturation effect is purely current induced and is associated with an increase of the electron concentration and electrical field at the active region/p-doped cladding layer interface. The result is that the electron current component that is directed from the active region to the p-doped clad layer increases. In the CW regime this current component increases additionally due to heating of the active region caused by the high current. Several publications (See, for example, an article entitled, “Effect of Heterobarrier Leakage on the Performance of High-Power 1.5 μm InGaAsP Multiple-Quantum Well Lasers” published in Journal of Applied Physics on pages 2211–2215 by Shterengas et al.) indicated that the increase of p-doping of the active region/p-doped clad interface allows the decreasing of the electron leakage due to the reduction of the interface electrical field. However, the increase of the hole concentration can lead to the decrease of laser efficiency caused by the additional optical losses associated with the strong photon absorption by free holes. In order to solve this problem, a broad waveguide (BW) design was suggested in U.S. Pat. No. 5,818,860, entitled “High Power Semiconductor Laser Diode,” issued Oct. 6, 1998, to Garbuzov, the inventor herein. In the case of BW lasers, 99% of the optical mode is confined in the broadened waveguide with total thickness of about 1 μm. The waveguide material is undoped thus providing minimum optical losses for lasing mode. The broad waveguide design was very useful for the fabrication of high power, high efficient diode lasers for numerous but non-telecom applications. The drawback of broad waveguide lasers for telecom applications is the large beam divergence in the direction perpendicular to the laser plane (fast direction). For 1.5 μm lasers with a 1 μm thick waveguide this divergence exceeds 40 degrees at half maximum intensity (FWHM>40°). This value of divergence is not compatible with effective laser coupling in single mode optical fiber. Methods of improving fast axis divergence are well known. For example, one method is to decrease the waveguide thickness down to 10–30 nm. However in the case of such narrow (NW) waveguide structures, a considerable portion (about 40%) of lasing mode propagates in the p-doped clad layer and optical losses caused by free hole absorption is high. As discussed above, the decrease of p-clad doping is not desirable because of electron leakage enhancement.
Thus, there is a need for a new design of telecom MQW low divergence lasers (especially pump lasers for fiber amplifiers) that provide high efficiency device operation at high current densities and elevated temperatures.